A number of printers, copiers, and multi-function products utilize heater chips in their printing heads for discharging ink drops. The ink is supplied through one or more ink vias in the chip. These heater chips typically provide only one heater array for each ink via that is disposed along one side of the ink via. In particular, as shown in FIG. 1, a traditional heater chip 100 may include three ink vias—a cyan ink via 102, a magenta ink via 104, and a yellow ink via 106. The cyan ink via 102 operates with the cyan heater array 108; the magenta ink via 104 operates with the magenta heater array 110; and the yellow ink via 106 operates with the yellow heater array 112.
Similarly, FIG. 2 shows a heater chip which includes three ink vias, each connected to a single heater array. The cyan ink via 202 operates with the cyan heater array 208; the magenta ink via 204 operates with the magenta heater array 210; and the yellow ink via 206 operates with the yellow heater array 212. However, the traditional use of single heater array on a single side of an ink via limits the achievable printing resolution, including the vertical resolution. The configurations shown in FIG. 1 and FIG. 2 may have significant difficulty providing ink drop sizes of less than 4 pL (picoliters) while achieving a vertical resolution of about 1200 dpi (dots per inch) or better. Therefore, it is desirable to position heater arrays on both sides of the ink vias, which allow the ink vias to provide smaller ink drops in order to achieve higher printing resolutions.
Additionally, for proper functionality, inkjet heater chips need to monitor and maintain the silicon substrate of the heater chip at an acceptable temperature for printing. If the temperature is too low, the ink drops formed will be smaller and have a lower drop-weight than that required for good image quality. As the temperature rises, the drop-weight of the ink drop will rise. Variations in drop-weight will cause visible hue shifts in the printed image.
A thermal sense resistor (TSR) is typically used to sense the temperature of the silicon substrate. The temperature of the heater chip shown in FIG. 1 is measured by way of a metal serpentine temperature sense resistor 120. The serpentine temperature sense resistor 120 is routed around the periphery of the heater chip and provides an average temperature of the entire die. This average measurement provides no discrimination between individual colors and does not provide any feedback on temperature differences between one area of the heater chip versus another. Thus, the metal serpentine temperature sense resistor 120 lacks the ability to control temperature on a per color or area basis.
The heater chip shown in FIG. 2 improves on that of FIG. 1 by providing for temperature sensing on a per color basis. Three temperature sense resistors 220, 222, and 224 are placed in close proximity to each of the ink vias, each situated on the same side of their respective ink vias. As shown, a first TSR 220 is situated on the left side of the cyan ink via 202 and cyan heater array 208; a second TSR 222 is situated on the left side of the magenta ink via 204 and magenta heater array 210; and a third TSR 224 is situated on the left side of the yellow ink via 206 and the yellow heater array 212. The ink vias 202, 204, and 206 act as a thermal barrier between the colors. All the thermal heater arrays 208, 210, and 212 are situated on only one side of their respective ink vias, ensuring that there is only a small amount of thermal crosstalk between the temperature sense resistors.
Once the temperature within the heater chip is measured, the temperature can be maintained and regulated at an acceptable temperature for printing. Some traditional heater chips use substrate heating elements to heat the silicon substrate to an acceptable temperature. Other heater chips apply fire pulses to selected heater arrays of a short duration to maintain desired temperature. The duration of the fire pulses is too short to cause the nucleation and subsequent ejection of an ink drop, but the pulses are sufficient to ensure that the heater chip operates within an acceptable temperature range. In FIG. 2, fire pulses may be applied on a per color basis from the respective heater arrays 208, 210, and 212. As previously mentioned, the ink vias 202, 204, and 206 function as thermal barriers between the colors. For example, heat generated by the magenta heater array 210 will not readily couple to the cyan heater array 208 and yellow heater array 212 on either side across the intervening ink vias 202 and 204. Thus, an adequate operating temperature can be maintained for each color of the heater chip.
When a heater array is positioned on both sides of ink vias, the temperature sensing and regulating devices utilized in the prior art do not provide adequate thermal control. A serpentine temperature sense resistor 120, as depicted in FIG. 1 is not capable of discriminating between the individual colors of the heater arrays and does not provide any feedback on temperature difference between various areas of the heater chip. Further, monitoring and regulating the operating temperature on a per color basis by situating a temperature sense resistor on the same side of each respective ink via, as shown in FIG. 2, is insufficient due to the fact that heater arrays of more than one color now occupy the silicon region between ink vias. Without accurate temperature readings, the method of providing fire pulses to regulate thermal conditions on a per color basis would also be subject to error.
Accordingly, there is a need in the industry for heater chips that can provide for monitoring and regulating the various regions of a heater chip at a desired temperature when heater arrays are placed on both sides of the ink vias.